1. Field of the Invention
The present invention relates to a capacitive ultrasonic transducer cell.
2. Description of the Related Art
The ultrasonic transducer cell has a function of converting an electric signal into an ultrasound and transmitting the ultrasound to an object, or a function of receiving the ultrasound reflected by the object and converting the received ultrasound into an electric signal.
More specifically, the ultrasonic transducer cell is configured by including a pair of plate-like electrodes (parallel plate electrodes) arranged opposite to each other so as to sandwich a cavity. Further, one of the pair of electrodes is included in a vibration membrane, so that an ultrasound is transmitted and received by vibration of the vibration membrane. When receiving an ultrasound, the ultrasonic transducer cell converts the ultrasonic signal into an electric signal on the basis of a change in the electrostatic capacitance between the pair of electrodes. Thus, in the case of an ultrasonic transducer cell of a conventional type, it is necessary to apply a DC bias voltage to the pair of electrodes especially at the time of reception.
In Japanese Patent Laid-Open No. 2-52599, there is disclosed an ultrasonic transducer cell which eliminates the need to apply the DC bias voltage by providing an electret membrane between the pair of electrodes.
The sound pressure of an ultrasound transmitted by the ultrasonic transducer cell depends on the electrostatic capacitance between the pair electrodes. FIG. 17 shows an equivalent circuit diagram of the ultrasonic transducer cell disclosed in Japanese Patent Laid-Open No. 2-52599. The ultrasonic transducer cell disclosed in Japanese Patent Laid-Open No. 2-52599 is configured by providing an electret film 503 between a pair of electrodes 501 and 502. In this case, the composite electrostatic capacitance C1 between the pair of electrodes 501 and 502 becomes a value obtained by combining an electrostatic capacitance Cmem of an insulating film 506, an electrostatic capacitance Ccav of a cavity 504 and an electrostatic capacitance Cele of the electret film 503.
Here, the thickness of the cavity 504 is determined by a thickness required for the insulating film 506, and hence is fixed regardless of the presence of the electret film 503. For this reason, the composite electrostatic capacitance C0 between the electrodes 501 and 502 at the time when the electret film 503 is not provided between the electrodes 501 and 502, is larger than the composite electrostatic capacitance C1.
In other words, in the configuration of the conventional ultrasonic transducer cell, it is not possible to maintain the electrostatic capacitance while reducing the DC bias voltage.